José Almiro A. M. Castro

Hornification—its origin and interpretation in wood pulps

Although perfectly diagnosed in terms of the occurrence of physical changes, the hornification phenomenon, in its origin, has frequently been associated with the formation of irreversible or partially reversible hydrogen bonding in wood pulps or paper upon drying or water removal. Its characterisation has therefore been confusing and unsatisfactory. The authors propose that a sufficiently varied source of experimental data already exists to show that hornification is only a particular case of lactone bridge formation in lignocellulosic materials.

The unscented filter as an alternative to the EKF for nonlinear state estimation: a simulation case study

Abstract The Kalman filter has been widely used for estimation and tracking of linear systems since its formulation in 1960 due to its simplicity and robustness. In many chemical engineering applications the extended Kalman filter (EKF) is often used to deal with certain classes of nonlinear systems. In spite of that, designing an EKF for highly nonlinear processes is not a trivial task, particularly those involving highly exothermic reactions. In this work we compared the performance of a new technique, the unscented filter, with that of the extended Kalman filter. The unscented filter produced better results without performing potentially ill-conditioned numerical calculations and linearly approximating the evolution of the state vector covariance.

Pear drying process analysis: Drying rates and evolution of water and sugar concentrations in space and time.

ABSTRACT Dried pears are produced in Portugal by a traditional solar drying process and, because they represent unquestionable regional economic interest, some studies are being conducted in order to convert their production to industrial drying methods. In this work the main mass transfer phenomena occurring during drying of pears are investigated. Experiments were carried out to determine the time evolution of the radial profiles of water and sugar content. The drying rates were also studied at constant drying temperatures of 30, 40 and 50°C. The results enable to conclude that the concentration profiles of water and sugar, both in space and time, follow an expected pattern if one takes into account the rates of water removal and the diffusion mechanisms. These also allow to explain why the sugar concentration increases in the periphery of the fruit. The drying rate curves obtained are typical, with the drying process at 30°C being more gradual but also much slower than that at 50°C.

Analysis of Moisture Content and Density of Pears During Drying

Abstract Solar dried pears of the “S. Bartolomeu” variety are a very much appreciated and preferred dried food product in Portugal. Nevertheless, the traditional solar drying is carried out at open air during the months of 07 and 08, and this nowadays is a disadvantage for larger productions. This work is to evaluate the possibility of producing dried pears from this and other varieties, maintaining the characteristics of the traditional dried pears. In this study four different types of pears were studied, including “S. Bartolomeu” as a basis for comparison and the drying method employed was the traditional one. From the results it was concluded that, although the behaviour of the four varieties do not vary significantly, one particular variety (“D. Joaquina”) is a good alternative to the “S. Bartolomeu” pear.

An analytical and experimental study of heat transfer in fixed bed

Abstract Heat transfer in a fixed-bed packed with polymeric adsorbent particles, percolated by a liquid phase flowing upward at 60 °C or downward at 20 °C, is experimentally studied in a column 9 cm in diameter and 1 m in length. The transient temperature profiles were measured axially and radially in the column. Sensitivity function calculations enabled to prove that the thermal parameters could be estimated from the experimental data and also to obtain information about the optimum location of the sensors. Experimental transient temperature responses were compared to those predicted by one-dimensional (1-D) and two-dimensional ( 2-D) pseudo-homogeneous (PH) models. By fitting the 2-D model solution to the experimental responses, values for the thermal parameter, effective radial and axial conductivities and the wall heat transfer coefficient were found.

Sensor Fault Detection and Identification in a Pilot Plant Under Process Control

The experimental evaluation of an automatic procedure for sensor fault detection and identification in a real process under closed-loop control is the objective of the present research. The scheme proposed here is very robust to faults in the main sensors of a multiloop control system, thus improving safety and reliability of plant operations. A state variable transformation is carried out in order to derive a model suitable for Recursive Least Squares (RLS) identification valid for all regimes of operation. The fault detection method is based on a moving window statistical analysis of the estimated model parameters. Simultaneously, a state estimation scheme, based on the Extended Kalman Filter (EKF), enables the fault identification, reduces false alarms and provides redundant measurements for alternative control purposes. Experimental runs were carried out in an industrial-scale pilot plant. Despite the large number of uncertainties and nonlinearities in the process, the system exhibited a good performance when faults occurred in the sensors of the control loops.

Two adaptive grid methods for fixed bed systems simulation

Abstract In this paper, two adaptive gridding algorithms, based on the Method of Lines, are proposed for the numerical solution of the PDE which describes the energy balance of a fixed bed system. A standard time-step control of the stiff solver—based on Richardson extrapolation—is coupled, in both algorithms, with a spatial adapting strategy. In one of the algorithms, this procedure consists of increasing the number of grid points where a certain measure of the error indicates that refinement is needed. In the other, the locally refined solutions are used to compute approximations for the spatial truncation error which are injected in the discretized equation. Both algorithms lead to highly accurate solutions on relatively coarse grids and present substantial savings in computing time and memory.

An RT-Linux based control system of a pilot plant for reaction kinetics and process control studies

Abstract The pilot plant under study is designed for determining the kinetics of the heterogeneous non-catalytic reaction of chemical pulping of wood and for teaching system dynamics and batch process control in an appropriate university course. The natura complexity of the system includes time varying dynamics and stepwise behavior and this makes it attractive for highlighting some important features of the real world of process control. Due to its specific design, the pilot is also very useful for kinetic studies of heterogenous reactions involving wood, ensuring a high level of the repeatability of results. The pilot plant and its control system have been used in teaching of chemical kinetics of heterogeneous reactions and are also part of the experimental program in a process control course.

Modelling multitubular catalytic reactors : the influence of shell side flow

Abstract The main purpose of the paper is to highlight some important features of modelling multitubular catalytic reactors. In this study a two dimensional model is used to describe the flow of the coolant on the shell side. The unit is divided in cells over the cross section of the assembly as well as in the axial direction. With this approach the coolant flow is made up of two different streams: one in pure crossflow perpendicular to the tubes and the other in parallel to the bundle. The performance of the tubes in each cell is computed with a two dimensional heterogeneous model and has shown to be sensitive to the number of cells used, particularly in the baffle window zones that are critical areas and therefore require a large number of cells. The influence of the operating conditions upon production and yield for the co-current and counter-current coolant flow regimes is examined and it is shown that carrying out the analysis on a single tube basis is not adequate to predict the optimum operating conditions.

A contribution to the study of runaway and parametric sensitivity in fixed bed catalytic reactors

Abstract The aim of the present work is to contribute to the analysis of parametric sensitivity in fixed bed reactors using an heterogeneous two dimensional model. The sensitivity functions are calculated along with the state variables and the bound of the region of parametric sensitivity is drawn by the maximum of the sensitivity of the particle temperature at the hot spot. The results show that with a two dimensional heterogeneous model runaway risk can be detected for operating conditions the one dimensional model qualifies as stable. This method can be applied to any reaction scheme, simple or complex. The runaway limit proved to be coincident whichever the parameter in question allowing to centre the analysis on simple operating parameters such as inlet gas concentration or temperature which are easily manipulated. In addition it highlighted the influence of the coolant temperature upon reactor stability and showed to be in good agreement with other criteria to foresee runaway.